When you stare at a colorful image and then turn to look at a neutral background, a “ghost image” appears in contrasting colors. Now, new research finds that a similar illusion occurs with shapes, turning circles into hexagons and vice versa.

Though similar, the two visual phenomena have different causes. While the color optical illusion, occurs because of tired-out light-sensing cells in the eye, the shape afterimage illusion arises from the visual parts of the brain, said study researcher Hiroyuki Ito, of Kyushu University in Japan.